System, method and apparatus for internal polarization rotation for horizontal cavity, surface emitting laser beam for thermally assisted recording in disk drive

ABSTRACT

A laser, such as a horizontal cavity surface emitting laser, with internal polarization rotation may be used in thermally assisted recording in hard disk drives. The desired polarization of the laser may be accomplished with two beam reflections off of facets within the laser. The facets may be formed in a single ion beam etching step. The laser may be used on a thermally assisted recording head to produce a polarized beam that is aligned with a track direction of the disk.

This divisional patent application claims priority to and the benefit ofU.S. patent application Ser. No. 12/346,930, filed Dec. 31, 2008, whichis incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

This disclosure relates in general to polarization rotation in lasersand, in particular, to an improved system, method and apparatus for alaser that may be suited for thermally assisted recording in hard diskdrives.

2. Description of the Related Art

Horizontal cavity, surface-emitting lasers, or HCSELs, have beendeveloped in recent years to combine some of the best properties ofconventional, end-firing diode lasers and vertical-cavitysurface-emitting lasers (VCSELs). Such lasers have an output that isgenerally polarized in the plane of the underlying wafer. Someapplications, however, require polarization rotation to properly orientthe laser polarization relative to the target.

For example, some implementations of thermally assisted recording (TAR)heads for hard disk drives require rotation of the polarized laser lightbeam. This step is useful since the normal polarization coming from thelaser is orthogonal to the polarization at the near field aperture usedin TAR. Rotating the polarization within the slider waveguide isaccomplished by structures that are very difficult to fabricate andgreatly complicate an already difficult fabrication process. Furthermorethe efficiency of polarization rotation for the very short devicelengths required in the slider remains to be demonstrated.

FIGS. 2A-2C depict a mirror-integrated laser diode 21, commonly referredto as a HCSEL. Laser diode 21 produces a beam 23 that is directedthrough the laser cavity and reflected downward by a facet 25 as areflected beam 27 toward a target. Importantly, the reflected beam 27 ispolarized (see reference arrow 33) in a direction that is orthogonal toa longitudinal direction of the laser diode 21.

For some applications, however, the polarization of the reflected laserbeam must be reoriented in a direction that aligns with the trackdirection. Examples of prior art techniques for reorienting a polarizedbeam include discrete components such as half-wave plates, non-lineareffects in fibers or active elements using Faraday or Kerr effects.Discrete components are not feasible for small scale applications due totheir large size and cost. Moreover, some techniques are wavelengthdependent. Mode-locked lasers often use nonlinearity-inducedpolarization rotation, which requires high power and is also prohibitivein terms of size and cost. Polarized continuous wave crystalline lasers(see, e.g., U.S. Pat. No. 3,914,710) induce thermal stress in a laserrod via optical pumping to cause polarization rotation. Although theseconventional solutions are workable in certain applications or withspecific devices, an improved design that works for HCSELs would bedesirable.

SUMMARY

Embodiments of a laser and method for forming the laser are disclosed.The laser may comprise a horizontal cavity, surface emitting laser(HCSEL) having a laser body configured to emit a beam that is reflectedby two facets before exiting the laser body for rotating a polarizationof the beam. The laser body may have a longitudinal axis along which thebeam is initially directed, and an initial polarization of the beam isorthogonal to the laser body and in a plane of an active layer of thelaser body. A first one of the two facets may initially reflect the beamin an orthogonal direction relative to the laser body and maintain theinitial polarization of the beam. A second one of the two facets mayreflect the beam from said first one of the two facets, and rotate thepolarization of the beam by 90 degrees.

In one embodiment, the first facet is normal to the active layer and atangle of 45 degrees relative to the laser beam. The second facet may beoriented at an angle of 45 degrees relative to the active layer andintersect the active layer in a line that is normal to incident laserbeam. The laser polarization may be initially in the plane of the activelayer and perpendicular to the propagation direction. After the firstfacet, the beam may be deflected by 90 degrees in the plane of theactive layer. The polarization is still in that plane, but is noworiented 90 degrees to the initial polarization direction. After thesecond facet, the beam may propagate normal to the active layer and thepolarization is still parallel to the active layer, but rotated 90degrees from its initial orientation. In this orientation it is alignedwith the direction of the tracks on the disk.

Moreover, in one embodiment, both etched facets in the HCSEL arefabricated simultaneously in a single etch process so that no additionalprocessing is required during manufacturing. This design allows rotationof the polarization without additional fabrication cost. The embodimentsoffer a much simpler and more cost effective solution than rotating thelaser beam within the slider waveguide or by other conventionalsolutions.

The foregoing and other objects and advantages of the presentembodiments will be apparent to those skilled in the art, in view of thefollowing detailed description of the present embodiments, taken inconjunction with the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the features and advantages of the presentembodiments are attained and can be understood in more detail, a moreparticular description of the embodiments briefly summarized above maybe had by reference to the embodiments thereof that are illustrated inthe appended drawings. However, the drawings illustrate only someembodiments of the embodiments and therefore are not to be consideredlimiting of its scope as the embodiments may admit to other equallyeffective embodiments.

FIG. 1 is a schematic isometric view of one embodiment of a thermallyassisted recording (TAR) device;

FIGS. 2A-2C are schematic top, side and end views, respectively, of alaser for a conventional TAR device;

FIGS. 3A-3C are schematic side and isometric views of an etchingprocess;

FIGS. 4A-4C are top, side and front end views, respectively, of oneembodiment of a laser for a TAR device;

FIG. 5 is a schematic isometric view of one embodiment of a TAR device,and single step etching process;

FIG. 6 is a schematic plan view of one embodiment of disk drive;

FIG. 7 is a schematic isometric view of a conventional laser diodestructure;

FIG. 8 is a schematic isometric view of a VCSEL structure;

FIG. 9 is a schematic sectional side view of a HCSEL structure; and

FIGS. 10-12 are coordinates that define geometries for embodiments of amethod of forming multiple facets in a HCSEL laser to rotatepolarization.

DETAILED DESCRIPTION

FIGS. 1, 3-6 and 8-12 depict embodiments of a laser and method forforming it. For example, FIG. 6 is a schematic drawing of one embodimentof an information storage system comprising the laser and a magnetichard disk file or drive 111 for a computer system. Drive 111 has anenclosure having a base 113 containing at least one magnetic disk 115.Disk 115 is rotated by a spindle motor assembly having a central drivehub 117. An actuator 121 comprises one or more parallel actuator arms125 in the form of a comb that is pivotally mounted to base 113 about apivot assembly 123. A controller 119 is also mounted to base 113 forselectively moving the comb of arms 125 relative to disk 115.

In the embodiment shown, each arm 125 has extending from it at least onecantilevered load beam and suspension 127. A magnetic read/writetransducer or head is mounted on a slider 129 and secured to a flexurethat is flexibly mounted to each suspension 127. The read/write headsmagnetically read data from and/or magnetically write data to disk 115.The level of integration called the head gimbal assembly is the head andthe slider 129, which are mounted on suspension 127. The slider 129 isusually bonded to the end of suspension 127. The head is typicallyformed from ceramic or intermetallic materials and is pre-loaded againstthe surface of disk 115 by suspension 127.

Suspensions 127 have a spring-like quality which biases or urges the airbearing surface of the slider 129 against the disk 115 to enable thecreation of the air bearing film between the slider 129 and disksurface. A voice coil 133 housed within a voice coil motor magnetassembly 134 is also mounted to arms 125 opposite the head gimbalassemblies. Movement of the actuator 121 (indicated by arrow 135) bycontroller 119 moves the head gimbal assemblies radially across trackson the disk 115 until the heads settle on their respective targettracks.

Horizontal cavity, surface-emitting lasers, or HCSELs, have beendeveloped in recent years to combine some of the best properties ofconventional, end-firing diode lasers and vertical-cavitysurface-emitting lasers (VCSELs). FIG. 1 depicts an example of animplementation of a thermally assisted recording (TAR) device 11 forwriting data to magnetic media disk 115. The slider body 129 includes acoil 17 and magnetic write pole 19. The mirror-integrated laser diode61, or HCSEL, is mounted to the slider body 129 and produces beam 63. Aswill be described herein, the beam 63 is directed through the lasercavity and reflected by facets toward the disk 115. The reflected beamis directed into a waveguide 29 and emitted from an optical near-fieldaperture 31 at track 43 having direction 45 on disk 115.

In conventional diode lasers (see, e.g., FIG. 7), the active layer 201is in the plane of the wafer. The gain region is formed by patternedcontacts on the top and bottom of the laser chip. A waveguide is formedby limiting the lateral extent of the gain region defined by thecontacts 203 (gain guiding) and/or by defining a region of highereffective index of refraction, e.g. by etching the chip to form a ridgewaveguide.

The end facets 205 on conventional diode lasers are formed by cleavingthe substrate 207. These facets can then be coated to adjust theirreflectivity, the higher the reflectivity the higher the Q of thecavity. This process can produce high quality lasers with low losses andlow threshold currents. It also can produce very high power lasers.These can be designed to emit in a single transverse mode which isimportant for many applications. The laser emission 209 emanates from anarea that is generally very small, on the order of a few microns acrossand is generally elliptical with the long axis in the plane of theactive layer 201. The emission is generally highly polarized with thepolarization direction being in the plane of the active layer and normalto the beam propagation direction. Since the laser emission comes fromthe edge of the laser device, these lasers are called edge-emittinglasers.

As shown in FIG. 8, another type of laser is the vertical-cavitysurface-emitting laser (VCSEL), which emits from an area 210 in adirection that is normal to the wafer surface, as the name implies. Thecavity is formed by a pair of Bragg reflectors 211 surrounding theactive region, which is generally a multi-quantum well structure 213.All these layers are parallel to the chip surface and are formed bydeposition processes. Electrical contacts 215, 217 are deposited on thetop and bottom surfaces of the chip to drive the laser. An advantage ofthe VCSEL over the conventional edge emitter is that VCSELs can betested at the wafer level since the emission surface 210 is on the wafersurface. VCSELs generally have a larger output area for the beam andthus a lower beam divergence than typical edge emitting lasers. Due tothe larger lateral extent of the cavity, these are generally multimodeemitters and not highly polarized.

The horizontal cavity, surface emitting laser or HCSEL (FIG. 9) is thelatest development in this evolution of solid-state diode lasers. Itcombines the ability to do wafer-level testing with a structure that isvery similar to the conventional edge emitting device. This enableshigh-power, single-mode lasers to be made with wafer-level testing. Thesurface emitting feature also enables some applications where thesurface emission facilitates integration with other devices. Like theedge-emitting laser, the HCSEL emission 821 is generally highlypolarized, with the polarization direction lying in the plane of theactive layer 823 between the contacts 825.

Many applications of laser diodes require a specific linearpolarization, typically one of two linear polarizations which, forpurposes of this disclosure, may be referred to as horizontal orvertical. Since the output polarization of a laser diode is set by thedevice physics, it cannot be easily adjusted if the normal outputpolarization does not match the desired polarization. The embodimentsdisclosed herein pertain to a means of rotating the output polarizationof a HCSEL device by 90 degrees so that they can be easily designed toprovide either horizontally or vertically polarized output.

An example of an application where such flexibility is important is inthe design of a laser for use in thermally-assisted magnetic recording.This application involves delivering the light from a laser to anear-field aperture that produces a very small illuminated spot on themagnetic disk in conjunction with a magnetic write field. A near-fieldaperture is required to produce an illuminated spot much smaller thanthe wavelength of the light, e.g. smaller than 100 nm. Examples of theseapertures are a “C” aperture and a bow tie aperture. These twoapertures, and near-field apertures in general, are very polarizationspecific. For optimum performance, these two apertures require lightpolarized along the direction of the magnetic track being written.

The HCSEL laser is well suited for the thermally assisted magnetic headbecause the laser can be piggy-backed on the magnetic slider as shown inFIG. 1. The front facet directs the light to the waveguide andnear-field aperture without requiring intervening optics. Given thedimensions of the typical slider and the HCSEL, the HCSEL will be mostfavorably mounted on the slider with the laser cavity running parallelto the magnetic track direction. However, with this arrangement, theoutput polarization will in general be in the cross-track direction.Although it is conceivable to rotate the polarization externally to thelaser, it would require additional components that add cost and bulk tothe overall assembly that would make the device impractical. Thus ameans of rotating the polarization by 90 degrees within the HCSELdevice, as presented herein, is needed.

Referring now to FIGS. 4A-4C, 5 and 6, one embodiment is shown as anapplication for a magnetic media disk in a hard disk drive. Although,the embodiment is shown in this type of application, it is not solimited but has many other applications as will be recognized by thosewith skill in the art. In the illustrated embodiment, a disk 115 hasmagnetic media formed in concentric tracks 43 (e.g., one shown) thatextend in a direction indicated by arrow 45. The slider 129 includes amagnetic write pole 47 for writing data to the disk 115. The slider 129also comprises a slider body 49 having an air bearing surface (ABS) 51,and a waveguide 53 with an optical near field aperture 55 adjacent thedisk 129.

A laser 61, such as a horizontal cavity, surface emitting laser (HCSEL),is mounted to the slider body 49 opposite the ABS 51. The laser 61 emitsa beam 63 that is directed through the waveguide 53 onto the disk 115for thermally assisted recording (TAR). The beam 63 is reflected by twofacets 65, 67 formed in the laser 61 before entering the waveguide 53for aligning the polarization of the beam 63 with the direction 45 ofthe concentric tracks 43.

In the embodiment shown, the beam 63 emitted by the laser 61 isinitially parallel to the disk 115 (see, e.g., FIG. 4A, the top view ofthe HCSEL) and the polarization 71 of the beam 63 is orthogonal to thedirection 45 of the concentric tracks 43. The first facet 65 reflectsthe initial beam 63 in an orthogonal direction 73 that is parallel tothe disk 115 but changes the polarization 75 of the beam to align withthe direction 45 of the concentric tracks 43. The second facet 67reflects the beam 73 from the first facet 65 toward the disk 115 (i.e.,along arrow 77 in FIGS. 4B and 4C), and maintains the new polarization75 of the beam. The second facet 67 also directs the beam 77 into thewaveguide 53 (FIG. 5), through the optical near field aperture 55, andonto the disk 115 adjacent the magnetic write pole 47. The beam 77 maybe directed onto the disk 115 directly beneath, in front of or behindthe magnetic write pole 47.

Embodiments of a method of forming facets in a laser also are disclosed.Two minor facets may be etched at different angles in a single step.Using technology such as reactive ion-beam etching, or RIBE, as used forforming the facet on a conventional HCSEL, two minors may be formed forpolarization rotation with two separate etch steps. However if thesecould both be made in a single etch step, the manufacturing costs wouldbe reduced and in fact would be essentially the same as for the currentsingle minor HCSEL process. This section describes how, by choosing theappropriate etch direction, two minors with different facet orientationscan be formed in a single etch step.

FIGS. 10-12 illustrate a coordinate system, including a z-axis along thelaser cavity and in the direction of propagation of the beam in thecavity. The y-axis is normal to the plane of the wafer and the x axis isin the plane of the wafer. The angles θ and φ need to be found thatdefine the desired orientation of the RIBE to simultaneously form thetwo mirrors for polarization rotation with their respective facetorientations. The angle θ is the angle between the y-axis and the RIBEdirection. The angle φ is the angle between the x-y plane and the planeformed by the RIBE axis and the y-axis.

Although the ion beam etching orientation or axis must lie in the planeof the mirror facet, it can impinge at any angle within that plane. Sothe ion beam axis is rotated out of the x-y plane by an angle φ as shownin FIG. 11. With this orientation, the plane formed by the ion beam axisand the y axis is normal to the wafer surface (the x-z plane). If a maskhas an edge along the projection of the ion beam axis into the x-z plane(e.g., the leftmost line denoted “r”), the ion beam etches a facet thatis normal to the surface and oriented at the angle φ from the x-y plane.The ion beam can be oriented at any angle θ in this plane, and the facetwill still be normal to the wafer surface. Thus, θ may be chosen toachieve the desired orientation of the second minor facet.

FIG. 12 shows how the orientation of the second facet is defined. If amask has an edge parallel to the line d, the line that projects the ionbeam axis back into the x-y plane and, thus, parallel to the z-axisforms a facet that makes an angle α with respect to the wafer surface.Although the ion beam axis does not lie along the dot-dashed line, itdoes lie in the plane formed by that line and the ion beam axis. Againthe freedom to orient the beam at any angle in this plane is beingexploited. With this, the expression for θ may be found, given thedesired φ and α. The final expression for θ is: θ=tan⁻¹ (1/(cos φ*tanα). Using this formula, if φ=α=45 degrees, then θ=54.7 degrees. Thisformula can be used to find the appropriate value of θ for any set ofvalues of φ and α.

In one embodiment, the method of forming facets in a laser comprisesproviding a laser diode 61 (FIGS. 3A-3C) having a longitudinal axis z, alateral axis x, and a transverse axis y. The laser diode 61 also has asurface 62 extending in a plane defined by the longitudinal and lateralaxes and perpendicular to the transverse axis. In a subsequent step, thesurface 62 of the laser diode 61 is masked 64 and two windows 66, 68(FIG. 3C) are provided in the mask 64. Thereafter, a single ion beam 70(e.g., reactive ion beam etching) is emitted at the masked laser diodeto simultaneously form two facets 65, 67 in the laser diode with thesingle ion beam through the two windows.

Comparing FIGS. 4A-4C, the first facet 65 is formed on the longitudinalaxis and oriented at an angle to the longitudinal and lateral axes butparallel to the transverse axis. The second facet 67 is laterally spacedapart from but transversely aligned with the first facet 65, parallel tothe longitudinal axis, and oriented at an angle to the lateral andtransverse axes.

The embodiments disclosed herein have several advantages, including alaser beam polarization rotation within the laser device withoutrequiring any external components. Unlike prior art devices, thefabricated laser diode structure is simple to fabricate and greatlyreduces complications during the fabrication process. This design alsois quite efficient at polarization rotation for the very short devicelengths required. Furthermore, it eliminates the need for half-waveplates, and avoids non-linear effects in fibers or active elements usingFaraday or Kerr effects. It also avoids the need for discretecomponents, is not wavelength dependent, offers a very low manufacturingcost and eliminates thermal stress.

While the embodiments disclosed herein have been shown or described inonly some forms, it should be apparent to those skilled in the art thatthey are not so limited, but are susceptible to various changes withoutdeparting from the scope of the disclosure.

1. A horizontal cavity, surface emitting laser (HCSEL), comprising: alaser body configured to emit a beam that is reflected by two facetsbefore exiting the laser body, and the two facets rotate a polarizationof the beam.
 2. A HCSEL according to claim 1, wherein the laser body hasa longitudinal axis along which the beam is initially directed, and aninitial polarization of the beam is orthogonal to the laser body and ina plane of an active layer of the laser body.
 3. A HCSEL according toclaim 2, wherein a first one of the two facets initially reflects thebeam in an orthogonal direction relative to the laser body and maintainsthe initial polarization of the beam, and a second one of the two facetsreflects the beam from said first one of the two facets, and rotates thepolarization of the beam by 90 degrees.
 4. A HCSEL according to claim 1,wherein the laser body has a longitudinal axis along which the beam isinitially directed, and an initial polarization of the beam is alignedwith the laser body and out of a plane of an active layer of the laserbody.
 5. A HCSEL according to claim 4, wherein a first one of the twofacets initially reflects the beam in an orthogonal direction relativeto the laser body and rotates the initial polarization of the beam by 90degrees, and a second one of the two facets reflects the beam from saidfirst one of the two facets, and maintains the rotated polarization ofthe beam.
 6. A HCSEL according to claim 1, further comprising a harddisk drive (HDD) having a disk and a slider, and the HCSEL is mounted tothe slider.
 7. A HCSEL according to claim 6, wherein the slider has anair bearing surface (ABS), a magnetic write pole, and a waveguide withan optical near field aperture adjacent the disk; and the HCSEL isconfigured to emit a beam that is directed through the waveguide ontothe disk for thermally assisted recording (TAR), and the rotatedpolarization of the beam aligns with a track direction on the disk.
 8. AHCSEL according to claim 7, wherein the laser is mounted to the sliderbody opposite the ABS.
 9. A HCSEL according to claim 7, wherein the beamis directed onto the disk either in front of or behind the magneticwrite pole.
 10. A laser, comprising: a horizontal cavity, surfaceemitting laser (HCSEL) comprising a laser body configured to emit a beamon a longitudinal axis along which the beam is initially directed, thebeam is reflected by two facets before exiting the laser body, a firstone of the two facets initially reflects the beam in an orthogonaldirection relative to the laser body, a second one of the two facetsreflects the beam from said first one of the two facets, and apolarization of the beam is rotated by the laser body.
 11. A HCSELaccording to claim 10, wherein an initial polarization of the beam isorthogonal to the laser body and in a plane of an active layer of thelaser body, the first one of the two facets maintains the initialpolarization of the beam, and the second one of the two facets rotatesthe polarization of the beam by 90 degrees.
 12. A HCSEL according toclaim 10, wherein an initial polarization of the beam is aligned withthe laser body and out of a plane of an active layer of the laser body,the first one of the two facets rotates the initial polarization of thebeam by 90 degrees, and the second one of the two facets maintains therotated polarization of the beam.
 13. A HCSEL according to claim 10,further comprising a hard disk drive (HDD) having a disk and a slider,and the HCSEL is mounted to the slider.
 14. A HCSEL according to claim13, wherein the slider has an air bearing surface (ABS), a magneticwrite pole, and a waveguide with an optical near field aperture adjacentthe disk; and the HCSEL is configured to emit a beam that is directedthrough the waveguide onto the disk for thermally assisted recording(TAR), and the rotated polarization of the beam aligns with a trackdirection on the disk.
 15. A HCSEL according to claim 14, wherein thelaser is mounted to the slider body opposite the ABS.
 16. A HCSELaccording to claim 14, wherein the beam is directed onto the disk eitherin front of or behind the magnetic write pole.
 17. A laser, comprising:a horizontal cavity, surface emitting laser (HCSEL) having a laser bodyconfigured to emit a beam on a longitudinal axis along which the beam isinitially directed, an initial polarization of the beam is orthogonal tothe laser body, the beam is reflected by two facets before exiting thelaser body, a first one of the two facets rotates the initialpolarization of the beam by 90 degrees, and a second one of the twofacets reflects the beam from said first one of the two facets, andmaintains the rotated polarization of the beam.
 18. A laser according toclaim 17, further comprising a hard disk drive (HDD) having a disk and aslider, and the laser is mounted to the slider.
 19. A laser according toclaim 18, wherein the slider has an air bearing surface (ABS), amagnetic write pole, and a waveguide with an optical near field apertureadjacent the disk; and the laser is configured to emit a beam that isdirected through the waveguide onto the disk for thermally assistedrecording (TAR), and the rotated polarization of the beam aligns with atrack direction on the disk.
 20. A laser according to claim 19, whereinthe laser is mounted to the slider body opposite the ABS, and the beamis directed onto the disk either in front of or behind the magneticwrite pole.